Floating Point Absorber Research Articles

The role of size and inertia on the hydrodynamics of a self-reacting heave single point absorber wave energy converter

We propose a detailed numerical study of a self-reacting floating point absorber, resembling the Ocean Power Technologies device PB3 PowerBuoy, for a possible installation in the Adriatic Sea. A simplified model of the mentioned device, composed of a floater sliding along a reacting body, which supports the floater, is studied, reproducing and analysing the most complex nonlinear phenomena, such as the wave overtopping and the out-of-water motion of the floater. Three different sizes of the device have been simulated, each with three different masses of the floater, deriving the time evolution of the wave in the surrounding of the floater, its frequency dispersion and energy content, the generated exciting force, the wave overtopping over the floater, and the out-of-water motion of the floater. The results highlight that the linear response is favoured by small floater density and optimal floater thickness. Additionally, large-sized devices are characterized by linear, but dissipative, response, while the response of medium-sized devices is only apparently linear.

Wave energy converter with floating-point absorber and catenary mooring: dynamic coupling analysis

Mooring design for floating wave energy converters (WECs) is crucial for station maintaining, efficient power collection, and economic concerns. In order to study the dynamic response of the floating-point absorber under the coupling action of the catenary in regular waves, this research presents the numerical modeling of the floating-point absorber alone with a catenary mooring system. Hydrodynamic behavior of the floating-point absorber is analyzed with respect to wave height, wave period, and current velocity. From the computational fluid dynamics (CFD) results, it can be deduced that the wave height has a much more pronounced impact on the longitudinal motion properties of WEC, such as the longitudinal force and the surge motion, and essentially no impact on the vertical force and the heave motion. The dynamic performance of the WEC under small wave periods are quite different from those under large wave periods. The current velocity also significantly affects the hydrodynamic performance of the WEC. The larger current velocity brings strong nonlinearity for the forces of the WEC. Under the combination of waves and current, the WEC and its mooring system will achieve a dynamic balance.

Parametric analysis of a two-body floating-point absorber wave energy converter

In the evolution of floating-point absorber wave energy conversion systems, multiple-body systems are gaining more attention than single-body systems. Meanwhile, the design and operation factors affecting the performance of multiple-body systems are much greater than those of single-body systems. However, no systematic study has yet been presented. In this article, a theoretical model is proposed by using a coupled oscillator system consisting of a damper-spring system to represent a two-body system (the floating body and the reacting body). Dimensionless expressions for the motion response and wave power absorption efficiency are derived. With the newly developed model, we prove that an appropriately tuned two-body system can obtain a limiting power absorption width of L/2π (L is the incident wavelength) as much as a single-body system. The generic case of a two-body system is presented with numerical simulations as an example. The results show that increasing the damping coefficient can reduce the wave frequency at which the peak of power absorption efficiency occurs. Increasing stiffness can make the wave frequencies for high power absorption efficiency move to a higher frequency region and can also make the spectrum bandwidth for high power absorption efficiency become narrower. Further, we show that the two-body system can absorb more wave energy at low wave frequencies than the single-body system.

Numerical study of bean-float wave energy converter with float number parametrization using WEC-Sim in regular waves with the Levelized Cost of Electricity assessment for Indian sea states

Present study deliberates on the numerical analysis of a new, bean-shaped, multi-body floating wave energy converter (BFWEC) using an open-source time-domain modeling tool called WEC-Sim (Wave Energy Converter SIMulator). The proposed device is a directionally-insensitive, floating-point absorber with a set of bean-shaped floats attached circumferentially around a central cylinder or buoy (CB or CS), extracting the wave energy under heave degree due to the relative motion of the floats with CB. The shape and arrangement of the floats around the CB make the BFWEC different from other existing devices. Authors propose three different layouts of BFWEC with multiple floats, namely: 4, 6, and 8, to study the influence of float's number on the device's overall performance. All layouts are incorporated with a linear damping power take-off (PTO) system to estimate absorbed power where the influence of damping on the performance is studied. The frequency-dependent parameters required to initiate time-domain analysis are obtained from the boundary element method (BEM). The influence of multi-float interactions on performance is assessed over an isolated single float arrangement. The Levelized Cost of assessment (LCoE) of BFWEC is calculated for various sea states of the Arabian Sea and the Bay of Bengal.

Motion simulation and performance analysis of two-body floating point absorber wave energy converter

As an important structure for utilizing wave energy, the two-body floating point absorber (FPA) wave energy converter has a simple structure, high energy conversion rate, and wide frequency response range. In this paper, a heave motion model of two-body FPA wave energy converter was established. On this basis, the secondary development of AQWA software programming in Fortran language was conducted by reckoning in the power take-off (PTO) effect, and the motion simulation of the two-body FPA wave energy converter was realized. For the specific oscillating two-body FPA model, the study analysed the frequency domain hydrodynamic performance and the time domain motion characteristics of the device. The results showed that the numerical model was adequate to predict the motion performance of the two-body FPA. The maximum value of the heaving response amplitude operator (RAO) was reached with the frequency of 0.7–0.8 rad/s. As the wave period increased, the generated power of the two-body FPA initially increased and subsequently decreased. The stiffness coefficient has less effect on the generated power than the damping coefficient. The results and conclusions have reference significance for the development of new wave energy devices and the efficient and reliable utilization of wave energy.

A Study on the Effects of Wave Spectra on Wave Energy Conversions

This research work presents an investigation into the effects of wave spectra on energy conversion for wave energy converters and shows it may be important what theoretical spectrum should be used for a better assessment of a wave energy converter (WEC) in the given sea conditions in a proposed deployment site. To illustrate the problem and the solution, a slightly modified Reference Model 3 (RM3) self-referencing floating point absorber device is used for examining the effects of spectrum types on wave energy conversion. The compared wave spectra include the most used theoretical wave spectra, such as the standard JONSWAP and Bretschneider spectra, as well as the real sea spectrum from field measurements. From the analysis it is shown that the modified RM3 WEC extracts a similar amount of energy from the recorded sea conditions (measured at the AMETS site 2010) when the device is optimized to both the Bretschneider spectrum and the real sea spectrum while the use of the JONSWAP spectrum for optimization leads to an over-prediction in the annual energy production of 16.5%. This may be because, in many practical applications for wave energy development, the JONSWAP spectrum is often preferred by the developers for assessing the device power performance. However, the use of the narrower JONSWAP spectrum (compared to the Bretschneider spectrum) may lead to inaccurate optimization results and power performance data for the device. Therefore, using the correct wave spectrum shapes in the assessment and optimization of a device is suggested for a more accurate assessment and a better understanding of the overall power performance of the device.

Experimental and numerical investigations of a two-body floating-point absorber wave energy converter in regular waves

This paper presents experimental and numerical studies on the hydrodynamics of a two-body floating-point absorber (FPA) wave energy converter (WEC) under both extreme and operational wave conditions. In this study, the responses of the WEC in heave, surge, and pitch were evaluated for various regular wave conditions. For extreme condition analysis, we assume the FPA system has a survival mode that locks the power-take-off (PTO) mechanism in extreme waves, and the WEC moves as a single body in this scenario. A series of Reynolds-averaged Navier–Stokes (RANS) simulations was performed for the survival condition analysis, and the results were validated with the measurements from experimental wave tank tests. For the FPA system in operational conditions, both a boundary element method (BEM) and the RANS simulations were used to analyze the motion response and power absorption performance. Additional viscous damping, primarily induced by flow separation and vortex shedding, is included in the BEM simulation as a quadratic drag force proportional to the square of body velocity. The inclusion of viscous drag improves the accuracy of BEM’s prediction of the heave response and the power absorption performance of the FPA system, which agrees well with experimental data and the RANS simulation results over a broad range of incident wave periods, except near resonance in large wave height scenarios. Overall, the experimental and numerical results suggest that nonlinear effects, caused by viscous damping and interaction between waves and the FPA, significantly influence the system response and power absorption performance. Nonlinear effects were found to be particularly significant when the wave height was large and the period was near or shorter than the resonant period of the FPA system. The study is expected to be helpful for understanding the nonlinear interaction between waves and the FPA system so that the structure of the FPA can be adequately designed.

Experimental Assessment of the Performance of CECO Wave Energy Converter in Irregular Waves

The performance assessment of a wave energy converter (WEC) is a key task. Depending on the layout of the WEC system and type of power take-off (PTO) mechanism, the determination of the absorbed power at model scale involves several challenges, particularly when the measurement of PTO forces is not available. In irregular waves, the task is even more difficult due to the random character of forces and motions. Recent studies carried out with kinetic energy harvesters (KEH) have proposed expressions for the estimation of the power based only on the measured motions. Assuming that the WEC behaves as a KEH at model scale, the expressions for power estimation of KEHs have been heuristically adapted to WECs. CECO, a floating-point absorber, has been used as case study. Experimental data from model tests in irregular waves are presented and analyzed. Spectral analyses have been applied to investigate the WEC responses in the frequency domain and to derive expressions to estimate the absorbed power in irregular waves. The experimental transfer functions of the WEC motions demonstrated that the PTO damping is significantly affected by the incident waves. Based on KEH approach's results, absorbed power and PTO damping coefficients have been estimated. A linear numerical potential model to compute transfer functions has been also implemented and calibrated based on the experimental results. The numerical results allowed the estimation of combined viscous and losses effects and showed that although the KEH approach underestimated the absorbed power, qualitatively reproduced the WEC performance in waves.

Assessment of damping coefficients of power take-off systems of wave energy converters: A hybrid approach

The damping coefficient of the power take-off (PTO) system is a key parameter in the performance assessment of a wave energy converter (WEC). However, since in most WEC studies the focus is mainly on the absorbed power, damping estimation is generally overlooked on the assumption that a single constant coefficient can properly characterize the WEC's damping of a given configuration for all wave conditions. Recently, while analyzing the experimental tests of CECO, a floating-point absorber WEC, significant discrepancies were found among their experimental responses under different incident waves. Instead of attributing those differences to nonlinear hydrostatic or Froude-Krylov effects, it was hypothesized that variations in the PTO damping associated to incident waves was the main cause. This study presents the experimental evidences of that behavior for regular and irregular waves. Furthermore, a hybrid approach for the assessment of damping coefficients is proposed and applied to CECO's experimental responses. The results demonstrated that: a) damping coefficients were significantly affected by wave conditions; b) higher PTO damping coefficients were obtained for milder irregular waves than for rougher regular waves; c) the hybrid approach reliably and efficiently estimated the WEC power in regular and irregular waves.

An improved boundary element method for modelling a self-reacting point absorber wave energy converter

A numerical model based on a boundary element method (BEM) is developed to predict the performance of two-body self-reacting floating-point absorber (SRFPA) wave energy systems that operate predominantly in heave. The key numerical issues in applying the BEM are systematically discussed. In particular, some improvements and simplifications in the numerical scheme are developed to evaluate the free surface Green’s function, which is a main element of difficulty in the BEM. For a locked SRFPA system, the present method is compared with the existing experiment and the Reynolds-averaged Navier–Stokes (RANS)-based method, where it is shown that the inviscid assumption leads to substantial over-prediction of the heave response. For the unlocked SRFPA model we study in this paper, the additional viscous damping primarily induced by flow separation and vortex shedding, is modelled as a quadratic drag force, which is proportional to the square of body velocity. The inclusion of viscous drag in present method significantly improves the prediction of the heave responses and the power absorption performance of the SRFPA system, obtaining results excellent agreement with experimental data and the RANS simulation results over a broad range of incident wave periods, except near resonance in larger wave height scenarios. It is found that the wave overtopping and the re-entering impact of out-of-water floating body are observed more frequently in larger waves, where these non-linear effects are the dominant damping sources and could significantly reduce the power output and the motion responses of the SRFPA system.

DESIGN OF A TWO-BODY WAVE ENERGY CONVERTER BY INCORPORATING THE EFFECT OF HYDRAULIC POWER TAKE-OFF PARAMETERS

Today, more than 80% of world's energy is obtained from the fossil fuels. Therefore, the researchers are looking for new methods for exploitation of renewable energy resources. Sea waves are an important source of environmental energy which can be converted into energy needed for different purposes. In current study, a floating-point absorber wave energy converter is simulated and optimized by the authors. The system is modeled through a two-body floating-point absorber (FPA) with two degrees of freedom in the heave direction. The model is equipped with a hydraulic power take off (PTO) system aiming to study the performance of the two-body wave energy converter (WEC) in regular and irregular waves. The fluid and structural parameters of the point absorber have been calculated and used for the purpose of optimization. Then, the optimum PTO damping coefficient was obtained and applied to this optimum value for system modeling. Further, a simulation study was performed in order to analyze the hydrodynamics of the two-body FPA system and its power output in the actual operational wave climate. Therefore, this paper has illustrated the application of a hydraulic power take off (PTO) system, giving an integrated pattern for modeling and designing of a wave energy converter system which is based on the hydrodynamic, hydraulic and structural dynamic fields. The results indicated that this modified system is optimal.

CFD Simulations of Floating Point Absorber Wave Energy Converter Arrays Subjected to Regular Waves

In this paper we use the Computational Fluid Dynamics (CFD) toolbox OpenFOAM to perform numerical simulations of multiple floating point absorber wave energy converters (WECs) arranged in a geometrical array configuration inside a numerical wave tank (NWT). The two-phase Navier-Stokes fluid solver is coupled with a motion solver to simulate the hydrodynamic flow field around the WECs and the wave-induced rigid body heave motion of each WEC within the array. In this study, the numerical simulations of a single WEC unit are extended to multiple WECs and the complexity of modelling individual floating objects close to each other in an array layout is tackled. The NWT is validated for fluid-structure interaction (FSI) simulations by using experimental measurements for an array of two, five and up to nine heaving WECs subjected to regular waves. The validation is achieved by using mathematical models to include frictional forces observed during the experimental tests. For all the simulations presented, a good agreement is found between the numerical and the experimental results for the WECs’ heave motions, the surge forces on the WECs and the perturbed wave field around the WECs. As a result, our coupled CFD–motion solver proves to be a suitable and accurate toolbox for the study of fluid-structure interaction problems of WEC arrays.

Wave energy harvesting using nonlinear stiffness system

A nonlinear stiffness mechanism installed in a floating point absorber (FPA) in regular waves allowed for studying the influence of the nonlinear behavior on wave energy harvesting. Static analysis of nonlinear stiffness system and time domain numerical simulations based on Cummins’ equation evaluated the effects of power take-off (PTO) damping, system stiffness and geometry dimensions. The results may apply to the evaluation of the balance between energy harvesting performance and practical design limitations. The nonlinear stiffness system improved the efficiency of wave energy harvesting, increasing mean power, by both pushing up the natural period, and broadening resonance, therefore proving more competitiveness.

Optimized Latching Control of Floating Point Absorber Wave Energy Converter

There is an increasing demand for energy in today’s world. Currently main energy resources are fossil fuels, which will eventually drain out, also the emissions produced from them contribute to global warming. For a sustainable future, these fossil fuels should be replaced with renewable and green energy sources. Sea waves are a gigantic and undiscovered vitality asset. The potential for extricating energy from waves is extensive. To trap this energy, wave energy converters (WEC) are needed. There is a need for increasing the energy output and decreasing the cost requirement of these existing WECs. This paper presents a method which uses prediction as a part of the control scheme to increase the energy efficiency of the floating-point absorber WECs. Kalman Filter is used for estimation, coupled with latching control in regular as well as irregular sea waves. Modelling and Simulation results for the same are also included.

Study on a Floating Point Absorber Type Wave Energy Converter with One Line Tension Leg Mooring

Motions and load characteristics of a floating point absorber type wave energy converter (WEC) were investigated by tank test considering operation modes. Four typical operation modes of a movable float, namely fixed mode, free mode, controlled mode without power generation, and controlled mode with power generation, were examined by the WEC model with an AC servo motor and a rack and pinion mechanism as a power take-off system in the tank test. The test results show that the operation modes have little effects on surge and pitch motions of the WEC. The measured load, such as mooring tension and axial force on the WEC, on the contrary, were strongly affected by the operation modes. The measured bending moment of the WEC, however, was affected by the WEC surge and pitch motions rather than the operation modes. Additionally, a numerical simulation using a commercial code, the Orcaflex, was conducted, and the simulation results were compared with measured ones. The WEC motions and mooring tension of the simulation results were comparable to those of measured ones in regular waves. The simulated power spectrum density of surge, pitch and mooring tension qualitatively matched measured ones, but they slightly underestimated measured ones at peak wave frequency in irregular waves.

A linear synchronous generator with a power of 30 kW for wave-power engineering

A floating buoy absorber that allows direct conversion of wave energy into electricity using a linear generator is a simple, but promising, device for conversion of wave energy. Results of studies of direct conversion of wave energy using a moored floating point absorber and linear synchronous generator are considered in this paper. The average power and performance of a linear synchronous generator have been determined for a significant wave. An algorithm for calculations of equivalent frequencies and emf amplitudes has been suggested for a modular design of the linear synchronous generator. The analytical expressions and dependences to determine the number of pole pairs and modules, as well as the main dimensions of the machine, have been obtained. It has been concluded that it is necessary to increase the number of poles of a linear generator to obtain an efficient design with reduced weight and overall dimensions. Application of the concentrated winding and permanent magnets with a tangential position has been justified. The parameters of electromagnetic fields and the synchronous generator for different operational conditions have been determined by the finite-element method. The design data and parameters of the six-module, 16-pole SLGPM-30-16 permanent magnet linear generator with a rating power of 30 kW are considered. The design has been optimized by calculations of the electromagnetic field; as a result, a weight of the generator has been reduced and much higher specific ratings than those in similar developments and in other devices for converting ocean-wave energy have been obtained.

Parametric study of two-body floating-point wave absorber

In this paper, we present a comprehensive numerical simulation of a point wave absorber in deep water. Analyses are performed in both the frequency and time domains. The converter is a two-body floating-point absorber (FPA) with one degree of freedom in the heave direction. Its two parts are connected by a linear mass-spring-damper system. The commercial ANSYS-AQWA software used in this study performs well in considering validations. The velocity potential is obtained by assuming incompressible and irrotational flow. As such, we investigated the effects of wave characteristics on energy conversion and device efficiency, including wave height and wave period, as well as the device diameter, draft, geometry, and damping coefficient. To validate the model, we compared our numerical results with those from similar experiments. Our study results can clearly help to maximize the converter’s efficiency when considering specific conditions.

Characterization of loads on a hemispherical point absorber wave energy converter

Large-scale experiments were carried out using a hemispherical shaped point absorber with a diameter of one meter, equivalent to scale 1:5 of the Wavestar North Sea prototype. The purpose of the experiments is to determine the hydrodynamic and dynamic loads on the Wavestar wave energy converters. With a better understanding of the loads the structural expenses can be reduced, and thereby reduce the cost of energy. The set-up consists of the hemisphere connected through a rigid arm that pivots on a non-moving main structure, which in this case is the gantry of the wave basin. The pitch motion of the hemisphere arm assembly is controlled using a linear hydraulic actuator. Wave and motion induced loads on the floating point absorber in regular waves as well as extreme conditions are presented. Drag, inertia and slamming pressure are determined directly from the experiments. Radiation, diffraction and excitation moments are determined by inviscid boundary element numerical models.

Analytical investigation and experimental validation of an inverted cup float used for wave energy conversion

World energy demand is increasing at an alarming rate and producing electricity from alternative or renewable energy sources is becoming necessary. There are many technologies to extract electric energy from sea waves such as: the oscillating water column, the point absorber, the overtopping system and the bottom hinged system. Many researchers are focusing on modeling the floating point absorber, which is thought to be the most cost effective technology to extract energy from sea waves. This paper is mainly work on a new design of float and the analytical analysis of its performance. This float consists of two parts; a hollow cylinder and an inverted cup fixed to its bottom. The float is initially submerged in water with sufficient submergence float. Water rises up due to the wave action and the float will follow the water motion which reduces slamming of the float. When the water level drops, the water enclosed in the inverted cup is exposed to a negative pressure which help the float down to follow the water wave motion without slamming. In this work, an analytical model is used MATLAB SOFTWARE to simulate the system of energy conversion. Moreover, a comparison for this model of the simulation results with experimental data to validate the model.

A Design Outline for Floating Point Absorber Wave Energy Converters

An overview of the most important development stages of floating point absorber wave energy converters is presented. At a given location, the wave energy resource has to be first assessed for varying seasons. The mechanisms used to convert wave energy to usable energy vary for different wave energy conversion systems. The power output of the generator will have variations due to varying incident waves. The wave structure-interaction leads to modifications in the incident waves; thus, the power output is also affected. The device has to be stable enough to prevent itself from capsizing. The point absorber will give optimum performance when the incident wave frequencies correspond to the natural frequency of the device. The methods for calculating natural frequencies for pitching and heaving systems are presented. Mooring systems maintain the point absorber at the desired location. Various mooring configurations as well as the most commonly used materials for mooring lines are discussed. An overview of scaled modelling is also presented.